[scx-layered] add matcher for when tid is pid

[likewhatevs]

add a matcher for when tid is equal to pid, to enable filtering tasks by this

[likewhatevs]

@valentinandrei @htejun @hodgesds does this sound reasonable?

[hodgesds]

Is that different than the PPIDEquals/TGIDEquals match? I think we have most things covered in terms of matches.

[likewhatevs]

Is that different than the [PPIDEquals/TGIDEquals match]

This is for filtering threads to those where TGID=PID.

My thinking is we can have a config like the following:

Layer Matches:
CommPrefix: "a thread name for a thread where we only care about the main/first thread"
AND
TGIDEqualsPID: true

Unless I'm missing something, using what we have now, we would need to specify TGID/PID manually.

Edit:

scx/scheds/rust/scx_layered/src/bpf/main.bpf.c

Lines 1740 to 1741 in 4ab8f4b

	case MATCH_PPID_EQUALS:
	return p->real_parent->pid == match->ppid;

is close but I think that isn't exactly what we want.

[likewhatevs]

#1204 is an attempt at this.

I think this is needed in addition to MATCH_PPID_EQUALS due to a nuance in the semantics of PID/PPID/TGID across user space and kernel space (I can't find the stack overflow w/ the only diagram I've ever found to make sense of how this works atm.), but I am not 100% sure about that atm (i.e. it could be that MATCH_PPID_EQUALS is sufficient).

In any event, this would do what we expect (get the current group leader thread) regardless of any such nuance.